myGreeting :: String 
myGreeting = "hello" ++ " world!"

myGreeting' :: String 
myGreeting' = (++) "hello" " world!"

hello :: String 
hello = "hello"

world :: String 
world = "world!"

main :: IO ()
main = do
  putStrLn myGreeting
  putStrLn myGreeting'
  putStrLn secondGreeting
    where secondGreeting = 
            concat [hello, " ", world]